Li2O-Al2O3-SiO2 type transparent crystallized glass

ABSTRACT

In an Li 2 O—Al 2 O 3 —SiO 2  transparent crystallized glass with β-quartz solid solution precipitated as main crystals, the Li +  concentration ratio an at a position of 50 nm from the surface of the glass is equal to 0.80 or less where the Li +  concentration at a position of 2000 nm from the surface is supposed to be equal to 1.

TECHNICAL FIELD

This invention relates to a transparent crystallized glass for use in afront window or an inspection window of a combustion apparatus forburning oil, coal, gas, wood, or the like, i.e., a room heater, aheating furnace, an annealing furnace, and so on.

BACKGROUND ART

A front window of a room heater or the like serves to enhance aroom-heating effect by transmitting heat radiation emitted from a flameinside the heater to the outside and to visually increase warm feelingby allowing the flame to be visible. An inspection window serves toallow a burning condition of the flame to be observed from the outside.These windows must be resistant against a high temperature produced fromthe flame and heat shock at the ignition. Therefore, a material used inthe windows of such combustion apparatus is required to be transparentand to be low in thermal expansion and high in mechanical strength sothat the heat resistance and the heat-shock resistance are excellent.

At present, use is made of a borosilicate glass, a silica glass, and anLi₂O—Al₂O₃—SiO₂ transparent crystallized glass in the windows of theroom heater and the like. However, the borosilicate glass is notsufficient in heat resistance and heat-shock resistance. The silicaglass is excellent In thermal characteristics but is expensive. On theother hand, the Li₂O—Al₂O₃—SiO₂ transparent crystallized glass isexcellent in heat resistance and heat-shock resistance because thecoefficient of thermal expansion is small and the mechanical strength ishigh, and can be produced at a relatively low cost. Therefore, thisglass is widely used.

However, the Li₂O—Al₂O₃—SiO₂ transparent crystallized glass isdisadvantageous in that, when It is placed in a combustion atmosphere,an inner surface or a burning-side surface of the window is subjected tochemical corrosion to produce microcracks which result in considerabledecrease in transparency and mechanical strength.

The reason why the above-mentioned disadvantage is caused is as follows.In the combustion atmosphere of the combustion apparatus for burningoil, coal, gas, wood, or the like, SO_(x) produced from sulfur containedin such fuel is present and reacts with H₂O produced by combustion toproduce H₂SO₄. H⁺ions produced from H₂SO₄ cause an ion exchange reactionwith Li⁺ ions in the Li₂O—Al₂O₃—SiO₂ transparent crystallized glass toshrink the volume of crystals, resulting in occurrence of the cracks.

In order to avoid the above-mentioned disadvantage, use is made of atechnique of forming a coating film such as SiO₂ on the surface of theLi₂O—Al₂O₃—SiO₂ transparent crystallized glass. However, this techniqueis unfavorable because the production cost is increased. Alternatively,it is possible to suppress the occurrence of the ion exchange reactionwith the H⁺ ions by reducing the content of the Li⁺ ions in theLi₂O—Al₂O₃—SiO₂ transparent crystallized glass. However, there arises aproblem that the transparency is deteriorated and the coefficient ofthermal expansion is increased.

It is therefore an object of this invention to provide anLi₂O—Al₂O₃—SiO₂ transparent crystallized glass which will not producemicrocracks even if it is exposed to a H₂SO₄containing atmosphereproduced by a fuel such as oil, coal, gas, wood, or the like for a longtime.

DISCLOSURE OF THE INVENTION

As a result of various experiments, the present inventors have foundthat, by reducing the Li⁺ concentration on the surface of a crystallizedglass as compared with the inside, the occurrence of microcracksresulting from an ion exchange reaction with H⁺ ions can effectively besuppressed, and herein propose this invention.

Specifically, an Li₂O—Al₂O₃—SiO₂ transparent crystallized glassaccording to this invention comprises Li₂O, Al₂O₃, and SiO₂ withβ-quartz solid solution precipitated as main crystals and ischaracterized in that the Li⁺ concentration ratio at a position of 50 nmfrom the surface of the Li₂O—Al₂O₃—SiO₂ transparent crystallized glassis equal to 0.80 or less where the Li⁺ concentration at a position of2000 nm from the surface is supposed to be equal to 1.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph schematically showing the distribution of Li⁺concentration in an Li₂O—Al₂O₃ 13 SiO₂ transparent crystallized glassaccording to this invention; and FIG. 2 is a graph showing a firingschedule.

BEST MODE FOR EMBODYING THE INVENTION

Referring to FIG. 1, in an Li₂O—Al₂O₃—SiO₂ transparent crystallizedglass according to this invention, the Li⁺ concentration ratio (R) at aposition of 50 nm from the surface thereof is equal to 0.80 or less,preferably 0.70 or less where the Li⁺ concentration at a position of2000 nm from the surface is supposed to be equal to 1. If the Li⁺concentration at the surface is low, the crystal amount of β-quartzsolid solution precipitated as main crystals is reduced so that Li⁺ ionscontained in the crystals are reduced in amount. Therefore, even ifH₂SO₄ is contained in a combustion atmosphere, an ion exchange lessensto occur so that the occurrence of microcracks is suppressed.

On the other hand, if the Li⁺ concentration ratio at the depth of 50 nmexceeds 0.80, the above-mentioned effect becomes extremely small so thatthe occurrence of microcracks can not be prevented.

As described above, the effect of suppressing the occurrence ofmicrocracks is greater as the Li⁺ concentration in the vicinity of thesurface is lower. Furthermore, as a portion in which the Li⁺concentration is low is thicker, the effect is much greater.Specifically, it is desired that the distance (D) between the surfaceand a position at which the Li⁺ concentration ratio reaches 0.95 is 70nm or more, preferably, 100 nm or more where the Li⁺ concentration at aposition of 2000 nm from the surface is supposed to be equal to 1. Asthe above-mentioned distance is greater, the amount of Li⁺ ions on thesurface is smaller and the crystal amount of precipitated 6 -quartzsolid solution is smaller so that the microcracks are more hardlyproduced. If the distance is smaller than 70nm, the effect ofsuppressing the occurrence of microcracks is insufficient.

The crystallized glass of this invention may contain various componentsother than Li₂O, SiO₂, and Al₂O₃. For example, the crystallized glassmay contain Na₂O, K₂O, MgO, ZnO, BaO, or the like as a component forpromoting melting and controlling the coefficient of thermal expansion,TiO₂, ZrO₂, or the like as a nucleating agent, P₂O₅ as a component forpromoting nucleation, and As₂O₃, Sb₂O₃, SnO₂, Cl, SO₃, or the like as afining agent. Specifically, it is preferable that the crystallized glassconsists essentially of, by weight percent, 55-75% SiO₂, 15-30% Al₂O₃,2-4.8% Li₂O, 0-1% Na₂O, 0-1% K₂O, 0-5% MgO, 0-2% ZnO, 0-3.5% BaO, 0.1-5%TiO₂, 0-4% ZrO₂, 0-5% P₂O₅, 0-2.5% As₂O₃, 0-2.5% Sb₂O₃, and 0-2.5% SnO₂.In case where Na₂O and/or K₂O is contained, it is desired that the Na⁺concentration ratio at a position of 50 nm from the surface is equal to2.0 or more, preferably, 2.2 or more and the K⁺ concentration ratio at aposition of 50 nm from the surface is equal to 1.5 or more, preferably,2.0 or more where each of the Na⁺ concentration and the K⁺ concentrationat a position of 2000 nm from the surface is supposed to be equal to 1.Specifically, if the Na⁺ concentration and/or the K⁺ concentration isgreater than the above-mentioned ratio, the Li⁺ concentration at thesurface is lowered and the crystal amount at the surface is reduced sothat the occurrence of microcracks is easily suppressed.

In this invention, each of the Li⁺ concentration, the Na⁺ concentration,and the K⁺ concentration can be controlled to achieve desireddistribution of concentration by the use of various methods such as amethod of adjusting crystallizing conditions and a method of subjectingan uncrystallized green glass to an ion exchange. In either method, itis important to select an appropriate processing condition for eachglass composition because an optimum condition is different dependingupon the composition.

Hereinafter, description will be made about specific examples of thisinvention.

Table 1 shows examples (Samples Nos. 1-3) of this invention and acomparative example (Sample No. 4).

Each sample was prepared as follows. At first, an Li₂—Al₂O₃—SiO₂ glassconsisting of 65.5% SiO₂, 22.1% Al₂O₃, 4.2% Li₂O, 0.5% Na₂O, 0.3% K₂O,0.5% MgO, 1.9% TiO₂, 2.3% ZrO₂, 1.4% P₂O₅, and 1.3% As₂O₃ was obtainedby melting and forming. Thereafter, the glass was fired in an electricfurnace under the condition shown in Table 1 to be crystallized. Thus,the sample was obtained. The firing was carried out in the scheduleshown in FIG. 2, in which the glass was held at a first temperature fortwo hours and at a second temperature for one hour. Each of the Li⁺,Na⁺, and K⁺ concentration ratios (concentration at 50 nm /concentrationat 2000 nm) and the thickness of a surface portion (distance from thesurface to a position at which the Li⁺ concentration ratio reaches 0.95)was obtained from the distribution of concentration of each componentmeasured by SIMS (Secondary Ion Mass Spectroscopy). The identificationof precipitated crystal was carried out by X-ray diffraction. Herein,“β-Q.” in Table 1 represents precipitated β-quartz solid solution.

TABLE 1 Sample No. 1 2 3 4 Firing Condition First Temperature (° C.) 770780 800 720 Second Temperature (° C.) 850 840 830 890 Li⁺ ConcentrationRatio 0.65 0.45 0.28 0.85 Na⁺ Concentration Ratio 3.05 3.65 4.80 1.02 K⁺Concentration Ratio 3.80 4.60 5.50 1.00 Thickness of Surface Portion 250320 350 60 (nm) Main Crystal β-Q. β-Q. β-Q. β-Q. Microcrack ResistanceAccelerated Test absent absent absent present Mounted Test(days) >50 >50 >50 7

As shown in Table 1, the main crystal in each sample was precipitatedβ-quarts solid solution.

In each of the samples Nos. 1-3, the Li⁺ concentration ratio was0.28-0.65, the Na⁺ concentration ratio was 3.05-4.80, the K⁺concentration ratio was 3.80-5.50, the distance from the surface to aposition at which the Li⁺ concentration ratio reaches 0.95 was 250-350nm.

On the other hand, in the sample No. 4, the Li⁺ Concentration ratio was0.85, the Na⁺ concentration ratio was 1.02, the K⁺ concentration ratiowas 1.00, the distance from the surface thereof to a position at whichthe Li⁺ concentration ratio reaches 0.95 was 60 nm.

Next, each sample was evaluated for the microcrack resistance. Themicrocrack resistance was evaluated by two kinds of methods including anaccelerated test and a mounted test in a combustion apparatus.

The accelerated test was carried out as follows. At first, 20 ml ofsulfuric acid water solution having a concentration of 6 vol % waspoured into a beaker having a volume of 1l. Subsequently, a net wasarranged in the beaker and the sample was put on the net to be exposedto vapor of sulfuric acid. Thereafter, the beaker was untightly closedby a glass plate. Then, after heating at 320° C. for 30 minutes, thesample was taken out and the surface thereof was observed by amicroscope. In Table 1, the sample is labelled “absent” and representsif the cracks were not observed and observed, respectively.

In the mounted test, each sample of the crystallized glass was attachedto a front surface of a stove using light oil containing sulfur as afuel. Burning was continuously carried out in an ordinary conditionuntil microcracks were visually observed. In terms of the number of dayswhich have lapsed, evaluation was carried out.

As a result, each of the samples Nos. 1-3 as the examples of thisinvention did not produce the cracks in the accelerated test. In themounted test, the occurrence of microcracks was not observed for morethan 50 days. Thus, the microcrack resistance was good.

On the other hand, the sample No. 4 as the comparative example wasrevealed to be far inferior in both of the accelerated test and themounted test than each sample as the example of the invention and to bepoor in microcrack resistance.

From the above-mentioned facts, it is shown that the crystallized glassaccording to this invention is excellent in microcrack resistance.

As described above, the Li₂O—Al₂O₃—SiO₂ transparent crystallized glassof this invention does not produce microcracks even if it is exposed fora long period of time to the atmosphere containing H₂SO₄ produced by thefuel such as oil, coal, gas, and wood. Therefore, the crystallized glassis very effective as a material of a front window or an inspectionwindow of a combustion apparatus such as a room heater, a heatingfurnace, and an annealing furnace.

Industrial Applicability

As described above, the Li₂O—Al₂O₃—SiO₂ transparent crystallized glassof this invention is suitable for use as a material of a front window oran inspection window of a combustion apparatus such as a room heater, aheating furnace, and an annealing furnace.

What is claimed is:
 1. An Li₂O—Al₂O₃—SiO₂ transparent crystallized glasswith β-quartz solid solution precipitated as main crystals, wherein theLi⁺ concentration at a distance of 50 nm from the surface of the glassis at most 8/10 of the Li⁺ concentration at a distance of 2000 nm fromthe surface of the glass, wherein said Li₂O—Al₂O₃—SiO₂ transparentcrystallized glass further contains Na₂O, the Na⁺ concentration at adistance of 50 nm from the surface thereof is at least twice the Na⁺concentration at a distance of 2000 nm from the surface of the glass,wherein said Li₂O—Al₂O₃—SiO₂ transparent crystallized glass furthercontains K₂O, the K⁺ concentration at a distance of 50 nm from thesurface thereof is at least one and a half times the K⁺ concentration ata distance of 2000 nm from the surface of the glass.
 2. AnLi₂O—Al₂O₃—SiO₂ transparent crystallized glass as claimed in claim 1,wherein the distance between the surface of said Li₂O—Al₂O₃—SiO₂transparent crystallized glass and a position at which said Li⁺concentration ratio reaches 0.95 is 70 nm or more.
 3. An Li₂O—Al₂O₃—SiO₂transparent crystallized glass as claimed in claim 1, wherein saidLi₂O—Al₂O₃—SiO₂ transparent crystallized glass is used for a window of acombustion apparatus.